Improve logic of CLEANFILES+=${PROG_FULL}.{bc,ll}.

The build rule describing how to create ${PROG_FULL}.{bc,ll} is only
dependent on LLVM_LINK being defined, not on MK_DEBUG_FILES being "yes".
Move the addition of ${PROG_FULL}.{bc,ll} out of the conditional block
under .if ${MK_DEBUG_FILES} != "no" and up next to where the build
rules for ${PROG_FULL}.{bc,ll} are defined.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12703